Precipitation hardened Co-Ni based heat-resistant alloy and production method therefor

ABSTRACT

A precipitation hardened Co—Ni based heat-resistant alloy is composed of, all by weight, not more than 0.05% of C; not more than 0.5% of Si; not more than 1.0% of Mn; 25 to 45% of Ni; 13 to 22% of Cr; 10 to 18% of Mo or 10 to 18% of Mo+½W; 0.1 to 5.0% of Nb; 0.1 to 5.0% of Fe; and further at least one kind of 0.007 to 0.10% of REM; 0.001 to 0.010% of B; 0.0007 to 0.010% of Mg and 0.001 to 0.20% of Zr; the balance of substantially Co and inevitable impurities, and Co 3 Mo or Co 7 Mo 6  is precipitated in boundaries between a fine twin structure and a parent phase.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a precipitation hardened Co—Nibased heat-resistant alloy and to a production method therefor, and moreparticularly, relates to a precipitation hardened Co—Ni basedheat-resistant alloy in which Co₃Mo or Co₇Mo₆ is precipitated atboundaries between a fine twin structure and a parent phase. Thestructure is suitable for springs, bolts, etc., that are used in parts,such as engine exhaust systems and peripheral devices in gas turbines,which are exposed to high temperatures.

[0003] 2. Related Art

[0004] Conventionally, heat-resistant parts which are used in parts,such as engine exhaust systems and peripheral devices in gas turbines,that are exposed to high temperatures, are manufactured by usingNi-based super heat-resistant alloys such as Inconel X-750 (Ni: 73.0mass %, Cr: 15.0 mass %, Al: 0.8 mass %, Ti: 2.5 mass %, Fe: 6.8 mass %,Mn: 0.70 mass %, Si: 0.25 mass %, C: 0.04, Nb+Ta: 0.9 mass %) andInconel 718 (Ni: 53.0 mass %, Cr: 18.6 mass %, Mo: 3.1 mass %, Al: 0.4mass %, Ti: 0.9 mass %, Fe: 18.5 mass %, Mn: 0.20 mass %, Si: 0.18 mass%, C: 0.04 mass %, Nb+Ta: 5.0 mass %).

[0005] These Ni-based super-heat-resistant alloys are reinforced byprecipitating γ′ phase (Ni₃ (Al, Ti, Nb) and γ″ phase (Ni₃Nb). However,when these alloys are used for long-periods at high temperatures at orabove 600° C., the γ′ phase and γ″ phase become coarse due to overaging,thereby causing a decrease in strength. Moreover, in parts such assprings and bolts on which stress is continuously applied, stressrelaxation is large, and thereby there is failure to maintain initialperformance originally required for the parts.

[0006] Therefore, the inventors of the present invention previouslydeveloped Co—Ni based heat-resistant alloys comprising, all by weight,not more than 0.05 mass % of C; not more than 0.5 mass % of Si; not morethan 1.0 mass % of Mn; 25 to 45 mass % of Ni; 13 to less than 18 mass %of Cr; 7 to 20 mass % of Mo+½W of at least one of Mo and W; 0.1 to 3.0mass % of Ti; 0.1 to 5.0 mass % of Nb; 0.1 to 5.0 mass % of Fe; and thebalance substantially of Co and inevitable impurities, the Co—Ni basedheat-resistant alloy, as necessary, further comprising: 0.007 to 0.10mass % of REM, further comprising, all by weight, at least one selectedfrom the group consisting of 0.001 to 0.010 mass % of B; 0.0007 to 0.010mass % of Mg; 0.001 to 0.20 mass % of Zr. The inventors also previouslydeveloped production methods for Co—Ni based heat-resistant alloys,comprising the steps of subjecting the alloy to a solid solution heattreatment at 1000 to 1200° C. or a hot working at this temperature, thensubjecting the alloy to a cold working or a warm working having areduction ratio of not less than 40% and then subjecting the alloy to anaging heat treatment at 500 to 800° C. for 0.1 to 50 hours. Theseinventions are disclosed in Japanese Unexamined Patent application(KOKAI) Publication No. 2002-97537.

[0007] In the Co—Ni based heat-resistant alloys, Cr which precipitatesas a σ phase is at least needed, solute elements such as Mo, Fe, and Nb,which are segregated in stacking faults of extended dislocation to blockdislocation movements, are increased to achieve high work hardeningperformance. These alloys have higher strengths at room temperature andcan inhibit decrease in strength even after long-periods of use underhigh temperatures in comparison with conventional Ni-basedsuper-heat-resistant alloys.

SUMMARY OF THE INVENTION

[0008] Therefore, objects of the present invention are to provide a heatresistant alloy which exhibits higher strength than the above-mentionedNi-based super-heat-resistant alloy and which can inhibit decrease instrength even after a long-period of use under high temperatures, and toprovide a production method therefor.

[0009] In order to solve the above-mentioned problems, the inventors ofthe present invention have carried out various research and studies onthe composition and aging heat treatment conditions of the Co—Ni basedheat-resistant alloys which exhibit higher strengths than theabove-mentioned Ni-based super-heat-resistant alloy, and can inhibitdecrease in strength even after a long-period of use under hightemperatures. As a result, the inventors found that when a Co—Ni basedheat-resistant alloy is subjected to an aging heat treatment underconditions of applying stress or high temperature, a fine twin structurehaving an average grain size of several microns is formed, and Co₃Mo orCo₇Mo₆ with sizes from several micron to several tens of nanometers isprecipitated in boundaries between the fine twin structure and a parentphase (refer to FIG. 1 and FIG. 2 showing structure photographs ofPractical Example 22 of the present invention). The inventors also foundthat when the above-mentioned structure is formed, a heat-resistantalloy which has high strength and which can inhibit decrease in strengtheven after a long-period of use under high temperatures can be obtained.The inventors also found that when Co—Ni based heat-resistant alloy isfirst subjected to a cold working or a warm working having a reductionratio of not less than 40% after a solid solution heat treatment and issecondly subjected to an aging heat treatment, a dislocation with highdensity is formed in a matrix by the cold working or the warm working,whereby strength under high temperatures is improved by anchoring thedislocation by precipitates formed by an aging heat treatment after thesolid solution heat treatment. Furthermore, a solute element such as Mois segregated in stacking fault surfaces of dislocation, and thedislocation is anchored. Therefore, an improvement effect in thestrength at room temperature and under high temperatures is obtained.

[0010] Moreover, the inventors found that in order to form a fine twinstructure having an average grain size of several microns, and to formfine precipitates such as Co₃Mo or Co₇Mo₆ having grain size of fromseveral microns to several tens of nanometers in boundaries between thefine twin structure and a parent phase, an aging heat treatment isperformed in which heat-resistant alloy is heated in an adequate time toa temperature of 600 to 800° C. in a condition of applying stress afterthe solid solution heat treatment. Alternatively, a working and an agingheat treatment is performed in which a heat-resistant alloy is firstsubjected to a cold working or a warm working having a reduction ratioof not less than 40% after a solid solution heat treatment and issecondly heated in an adequate time at a temperature of 600 to 800° C.in a condition of applying stress. Alternatively, a working and an agingheat treatment is performed in which a heat-resistant alloy is firstsubjected to a cold working or a warm working having a reduction ratioof not less than 40% after a solid solution heat treatment and issecondly heated in an adequate time at a temperature of 800° C. to 950°C.

[0011] The present invention has been made based on these findings. Inthe following explanation, “%” refers to mass %.

[0012] The present invention provides a precipitation hardened Co—Nibased heat-resistant alloy comprising, all by weight, not more than0.05% of C; not more than 0.5% of Si; not more than 1.0% of Mn; 25 to45% of Ni; 13 to 22% of Cr; 10 to 18% of Mo or 10 to 18% of Mo+½W; 0.1to 5.0% of Nb; 0.1 to 5.0% of Fe; if any 0.1 to 3.0% of Ti; at least onekind of 0.007 to 0.10% of REM; 0.001 to 0.010% of B; 0.0007 to 0.010% ofMg and 0.001 to 0.20% of Zr; the balance of Co and inevitableimpurities; a fine twin structure; a parent phase; and Co₃Mo or Co₇Mo₆is precipitated at boundaries of the fine twin structure and the parentphase.

[0013] In another aspect of the invention, the invention provides aproduction method for precipitation hardened Co—Ni based heat-resistantalloy, the method comprising the steps of: preparing an alloycomprising, all by weight, not more than 0.05% of C; not more than 0.5%of Si; not more than 1.0% of Mn; 25 to 45% of Ni; 13 to 22% of Cr; 10 to18% of Mo or 10 to 18% of Mo+½W; 0.1 to 5.0% of Nb; 0.1 to 5.0% of Fe;if any 0.1 to 3.0% of Ti; at least one kind of 0.007 to 0.10% of REM;0.001 to 0.010% of B; 0.0007 to 0.010% of Mg and 0.001 to 0.20% of Zr;the balance of Co and inevitable impurities; subjecting the alloy to asolid solution heat treatment; and subjecting the alloy to an aging heattreatment at 600 to 800° C. for 0.5 to 16 hours in a condition ofapplying stress, thereby forming a fine twin structure in a parentphase, and precipitating Co₃Mo or Co₇Mo₆ at a boundary of the fine twinstructure and the parent phase.

[0014] Moreover, in another aspect of the invention, the inventionprovides a production method for precipitation hardened Co—Ni basedheat-resistant alloy, the method comprising the steps of: preparing analloy comprising, all by weight, not more than 0.05% of C; not more than0.5% of Si; not more than 1.0% of Mn; 25 to 45% of Ni; 13 to 22% of Cr;10 to 18% of Mo or 10 to 18% of Mo+½W; 0.1 to 5.0% of Nb; 0.1 to 5.0% ofFe; if any 0.1 to 3.0% of Ti; at least one kind of 0.007 to 0.10% ofREM; 0.001 to 0.010% of B; 0.0007 to 0.010% of Mg and 0.001 to 0.20% ofZr; the balance of Co and inevitable impurities; subjecting the alloy toa solid solution heat treatment; subjecting the alloy to a cold workingor a warm working having a reduction ratio of not less than 40%; andsubjecting the alloy to an aging heat treatment at 600 to 800° C. for0.5 to 16 hours in a condition of applying stress, thereby forming afine twin structure in a parent phase, and precipitating Co₃Mo or Co₇Mo₆at a boundary of the fine twin structure and the parent phase.

[0015] Furthermore, in another aspect of the invention, the inventionprovides a production method for precipitation hardened Co—Ni basedheat-resistant alloy, the method comprising the steps of: preparing analloy comprising, all by weight, not more than 0.05% of C; not more than0.5% of Si; not more than 1.0% of Mn; 25 to 45% of Ni; 13 to 22% of Cr;10 to 18% of Mo or 10 to 18% of Mo+½W; 0.1 to 5.0% of Nb; 0.1 to 5.0% ofFe; if any 0.1 to 3.0% of Ti; at least one kind of 0.007 to 0.10% ofREM; 0.001 to 0.010% of B; 0.0007 to 0.010% of Mg and 0.001 to 0.20% ofZr; the balance of Co and inevitable impurities; subjecting the alloy toa solid solution heat treatment; subjecting the alloy to a cold workingor a warm working having a reduction ratio of not less than 40%; andsubjecting the alloy to an aging heat treatment at 800° C. to 950° C.for 0.5 to 16 hours, thereby forming a fine twin structure in a parentphase, and precipitating Co₃Mo or Co₇Mo₆ at a boundary of the fine twinstructure and the parent phase.

[0016] In the precipitation hardened Co—Ni based heat-resistant alloy ofthe present invention, fine precipitates are formed at boundariesbetween the fine twin structure and a parent phase. The precipitates arenot grown to be coarse at high temperatures of about 700° C., an effecton anchoring dislocation is performed even at high temperatures of notless than 700° C. due to interaction between the precipitates and thedislocation. The precipitates are formed in grain boundaries of a finetwin structure having average grain size of several microns. Therefore,the precipitates suppress grain boundary sliding as a obstacle when thegrain boundary moves at high temperatures of not less than 700° C., andprevents coursening of the grains. Accordingly, high strength, such ascreep strength, is excellent.

[0017] Moreover, in the production method of the precipitation hardenedCo—Ni based heat-resistant alloy of the present invention, theheat-resistant alloy is subjected to an aging heat treatment for 0.5 to16 hours at a temperature of 600 to 800° C. in a condition of applyingstress after a solid solution heat treatment by heating at 1000 to 1200°C. Alternatively, the heat-resistant alloy is first subjected to a coldworking or a warm working having a reduction ratio of not less than 40%after the solid solution heat treatment and is secondly subjected to anaging heat treatment for 0.5 to 16 hours at a temperature of 600 to 800°C. in a condition of applying stress. Alternatively, the heat-resistantalloy is first subjected to a cold working or a warm working having areduction ratio of not less than 40% after the solid solution heattreatment and secondly to an aging heat treatment by heating for 0.5 to16 hours at temperature of 800° C. to 950° C. Therefore, the fine twinstructure can be formed and at least one kind of Co₃Mo and Co₇Mo₆ can beprecipitated in boundaries between the fine twin structure and a parentphase.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a scanning electron micrograph of drawing substitutionshowing a structure magnified 5000 times of Practical Example No. 22 ofthe present invention.

[0019]FIG. 2 is a scanning electron micrograph of drawing substitutionshowing a structure magnified 2000 times of Practical Example No. 22 ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERED EMBODIMENTS

[0020] Next, the following description will discuss the reasons for theabove-mentioned limitations to the composition in the precipitationhardened Co—Ni based heat-resistant alloy and the production method ofthe present invention.

[0021] C: Not More Than 0.05%

[0022] Carbon C is bound to Nb and Ti to form carbides to prevent grainsfrom becoming coarse at the time of a solid solution heat treatment, andalso to strengthen the grain boundary; thus, this element is containedfor these purposes. In order to obtain these effects, the content mustbe not less than 0.005%. However, since a content exceeding 0.05%, morespecifically, 0.03%, would cause decrease in the toughness and corrosionresistance, and would also form a carbide with a dislocation anchoringelement such as Mo, thereby resulting in interference with thedislocation anchoring, the content must not be more than 0.05%. Thepreferable range is 0.005 to 0.03%.

[0023] Si: Not More Than 0.5%

[0024] Since Si is effectively used as a deoxidizer, this element iscontained for this purpose. However, since a content exceeding 0.5%,more specifically, 0.3%, would cause decrease toughness, the content isnot more than 0.5%. The preferable range is not more than 0.3%.

[0025] Mn: Not More Than 1.0%

[0026] Since Mn is effectively used as a deoxidizer, and reducesstacking fault energy to improve the work hardening performance, thiselement is contained for this purpose. However, since a contentexceeding 1.0%, more specifically, 0.7%, would cause decrease incorrosion resistance, the content must not be more than 1.0%. Thepreferable range is not more than 0.7%.

[0027] Ni: 25 to 45%

[0028] Since Ni is an element that is used for stabilizing austeniteserving as a matrix and improves heat resistance and corrosionresistance of the alloy, this element is contained for these purposes.In order to obtain these effects, the content must not be less than 25%,more preferably, 27%. However, since a content exceeding 45%, wouldcause decrease in work hardening performance, the content must be 25 to45%. The preferable range is 27 to 45%.

[0029] Cr: 13% to less than 22%

[0030] Since Cr is an element that is used for improving the heatresistance and corrosion resistance, this element is contained for thesepurposes. In order to obtain these effects, the content must not be lessthan 13%, more preferably, 16%. However, since a content exceeding 22%,more specifically, 21%, tends to cause precipitation of a a phase, thecontent must be in a range of 13 to 22%. The preferable range is 16 to21%.

[0031] Mo+½W: 10 to 18%

[0032] Since Mo and W are solid solution-treated into the matrix andstrengthen the matrix to improve the work hardening performance, theseelements are contained for these purposes. In order to obtain theseeffects, the content must not be less than 10%, more preferably, 11%,and preferably the Mo content must not be less than 8.0% in the case ofcontaining Mo and W. However, since when the total amount of the contentof Mo and ½ of the content of W exceeds 18%, precipitation of a σ phasetends to occur, the content must be in a range of 10 to 18%. Thepreferable range is 11 to 18%.

[0033] Nb: 0.1 to 5.0%

[0034] Nb is bound to C to form carbides to prevent grains from becomingcoarse in a solid solution heat treatment and to strengthen the grainboundary, and also solid solution-treated in the matrix to strengthenthe matrix, thereby improving the work hardening performance. Thus, thiselement is contained for these purposes. In order to obtain theseeffects, the content must not be less than 0.1%, more preferably, 0.8%.However, since the content exceeding 5.0%, more specifically, 3.0%,would cause precipitation of a a phase (Ni₃Nb) resulting in decrease inworkability and toughness, the content must be in a range of 0.1 to5.0%. The preferable range is 0.8 to 3.0%.

[0035] Fe: 0.1 to 5.0%

[0036] Since Fe is solid solution-treated in the matrix to strengthenthe matrix, this element is contained for this purpose. In order toobtain this effect, the content must not be less than 0.1%, and morepreferably, 0.5%. However, since a content exceeding 5.0%, morespecifically, 4.8%, causes decrease in oxidation resistance property,the content must be in a range of 0.1 to 5.0%. The preferable range is0.5 to 4.8%.

[0037] The application of Mo, Nb, and Fe in a combined manner makes itpossible to greatly increase the solid solution strength and workhardening of the matrix, which greatly enhances the maximum tensilestrength obtained at room temperature and at high temperatures, andexerts an effect of shifting the temperature having a maximum of thetensile strength at a high temperature to the high temperature side, incomparison with the application of Mo and Nb or Mo and Fe in a combinedmanner.

[0038] Ti: 0.1 to 3.0%

[0039] Since Ti improves strength, this element is contained for thispurpose. In order to obtain this effect, the content must not be lessthan 0.1%, more preferably, 0.5%. However, since a content exceeding3.0%, more specifically, 2.5%, would cause precipitation of an T phase(Ni₃Ti) resulting in decrease in workability and toughness, the contentmust be in a range of 0.1 to 3.0%. The preferable range is 0.5 to 2.5%.

[0040] REM: 0.007 to 0.10%

[0041] Since REM, which is at least one rare-earth elements such as Y,Ce, and misch metal, improves the hot workability and oxidationresistance property, this is contained for these purposes. In order toobtain these effects, the content must not be less than 0.007%, morepreferably, 0.01%. However, since a content exceeding 0.10%, morespecifically, 0.04%, causes decrease in hot workability and oxidationresistance property in an inverse manner, the content must be in a rangeof 0.007 to 0.10%. The preferable range is 0.01 to 0.04%.

[0042] B: 0.001 to 0.010%, Mg: 0.0007 to 0.010%, Zr: 0.001 to 0.20%.

[0043] Since B, Mg, and Zr improve the hot workability and strengthenthe grain boundary, these elements are contained for these purposes. Inorder to obtain these effects, B must be 0.001%, more preferably,0.002%, Mg must be 0.0007%, more preferably, 0.001%, and Zr must be0.001%, more preferably, 0.01%. However, since B exceeding 0.010%, morespecifically, 0.006%, Mg exceeding 0.010%, more specifically, 0.004% andZr exceeding 0.20%, more specifically 0.05%, would cause decrease in hotworkability and oxidation resistance property, the ranges of thecontents must be respectively in the above-mentioned ranges. Morepreferably, B is in a range of 0.002 to 0.006%, Mg is in a range of0.001 to 0.004%, and Zr is in a range of 0.01 to 0.05%.

[0044] Co: Balance

[0045] Co, which has a close-packed hexagonal lattice structure, isallowed to contain Ni so as to have a face-centered cubic latticestructure, that is, austenite, thereby exerting a high work hardeningperformance.

[0046] The precipitation hardened Co—Ni based heat-resistant alloy ofthe present invention comprises the above-mentioned composition, and hasa structure in which Co₃Mo or Co₇Mo₆ is precipitated in boundariesbetween a fine twin structure and a parent phase.

[0047] Next, the following description will discuss the productionmethod of the precipitation hardened Co—Ni based heat-resistant alloy ofthe present invention. In the production method of the precipitationhardened Co—Ni based heat-resistant alloy of the present invention, afine twin structure having average grain size of several microns isformed in a precipitation hardened Co—Ni based heat-resistant alloyhaving the above-mentioned composition, Co₃Mo or Co₇Mo₆ of sizes fromseveral microns to several tens of nanometers is precipitated inboundaries between the fine twin structure and a parent phase, andthereby a heat-resistant alloy which has high strength and which caninhibit decrease in strength even after a long-period of use under hightemperatures can be obtained.

[0048] Therefore, the production method of the precipitation hardenedCo—Ni based heat-resistant alloy of the present invention ischaracterized in that the above-mentioned Co—Ni based heat-resistantalloy is first subjected to a solid solution heat treatment by heatingto 1000 to 1200° C., etc., and secondly to an aging heat treatment byheating for 0.5 to 16 hours at temperature of 600 to 800° C. in acondition of applying stress. Furthermore, another production method ofthe precipitation hardened Co—Ni based heat-resistant alloy of thepresent invention is characterized in that the above-mentioned Co—Nibased heat-resistant alloy is first subjected to a solid solution heattreatment, secondly to a cold working or a warm working having areduction ratio of not less than 40%, and thirdly to an aging heattreatment by heating for 0.5 to 16 hours to a temperature of 600 to 800°C. in a condition of applying stress. Moreover, another productionmethod of the precipitation hardened Co—Ni based heat-resistant alloy ofthe present invention is characterized in that the above-mentioned Co—Nibased heat-resistant alloy is first subjected to a solid solution heattreatment, secondly to a cold working or a warm working having areduction ratio of not less than 40%, and thirdly to an aging heattreatment by heating for 0.5 to 16 hours to a temperature of 800° C. to950° C. in an unloaded condition.

[0049] In the production method of the precipitation hardened Co—Nibased heat-resistant alloy of the present invention, the solid solutionheat treatment is performed in order to make the structure uniform andto lower the hardness to facilitate working. Therefore, the solidsolution heat treatment is preferably performed by heating to 1000 to1200° C. A temperature lower than 1000° C. fails to provide asufficiently uniform structure and also fails to lower the hardness,thereby causing difficulty in working. Furthermore, a temperature lowerthan 1000° C. might cause precipitation of a compound such as Mo thatexerts an anchoring effect on dislocations, and a subsequent reductionin the age hardening property. A temperature exceeding 1200° C. makescrystal grains coarse, resulting in decrease in toughness and strength.

[0050] In the production method of the precipitation hardened Co—Nibased heat-resistant alloy of the present invention, the heat-resistantalloy is subjected to an aging heat treatment by heating for 0.5 to 16hours to a temperature of 600 to 800° C. in a condition of applyingstress in order to form a fine twin structure having an average grainsize of several microns and to precipitate Co₃Mo or Co₇Mo₆ of sizes fromseveral microns to several tens of nanometers in boundaries between thefine twin structure and a parent phase. The applied stress in the agingheat treatment is preferably about 100 to 400 MPa. An applied stressless than 100 MPa fails to sufficiently precipitate fine Co₃Mo or Co₇Mo₆in boundaries between a fine twin structure and a parent phase. Theapplied stress exceeding 400 MPa results in saturation and transformsthe alloy which is subjected to the aging heat treatment.

[0051] In the production method of the precipitation hardened Co—Nibased heat-resistant alloy of the present invention, the heat-resistantalloy is subjected to an aging heat treatment by heating for 0.5 to 16hours at a temperature of 600 to 800° C. because a temperature lowerthan 600° C. or a time shorter than 0.5 hours fails to sufficientlyprecipitate a fine twin structure and fine Co₃Mo or Co₇Mo₆ in boundariesbetween the fine twin structure and a parent phase, and a temperaturehigher than 800° C. or a time longer than 16 hours results in saturationand makes the precipitates rather coarse, thereby causing decrease instrength, and this also causes greater creep elongation by causingdecrease in hardness and strength by causing the dislocation to reformwhen the aging heat treatment is additionally performed after performinga cold working or a warm working having a reduction ratio of not lessthan 40%.

[0052] In the production method of the precipitation hardened Co—Nibased heat-resistant alloy of the present invention, the heat-resistantalloy is subjected to a cold working or a warm working having areduction ratio of not less than 40% before an aging heat treatment in acondition of applying stress because forming dislocations at highdensity is necessary, and a density lower than 40% fails to formdislocations at high density. By an aging heat treatment after formingthe dislocations at high density, solute atoms such as Mo and Fe aresegregated in stacking faults formed between half-dislocations ofextended dislocations; thus, the dislocation movements are blocked sothat stress relaxation, that is, reoccurrence of dislocations, issuppressed. As a result, a heat-resistant alloy which has high strengthand which can inhibit decrease in strength even after a long-period ofuse under high temperatures can be obtained, combined with an effect inwhich a fine twin structure forms and fine Co₃Mo or Co₇Mo₆ precipitatesin boundaries between the fine twin structure and a parent phase.

[0053] In a production method of the precipitation hardened Co—Ni basedheat-resistant alloy of the present invention, the heat-resistant alloyis subjected to an aging heat treatment by heating for 0.5 to 16 hoursat a higher temperature of 800° C. to 950° C. after the cold working orwarm working having a reduction ratio of not less than 40% after thesolid solution heat treatment because a fine twin structure havingaverage grain size of several microns must be formed and Co₃Mo or Co₇Mo₆of sizes from several microns to several tens of nanometers must beprecipitated in boundaries between the fine twin structure and a parentphase. Although aging heat treatments are performed in a condition ofapplying stress in another production method of the precipitationhardened Co—Ni based heat-resistant alloy of the present invention, anaging heat treatment is performed at a higher temperature of 800° C. to950° C. instead of using the condition of applying stress in thisproduction method of the precipitation hardened Co—Ni basedheat-resistant alloy of the present invention. In this productionmethod, the aging heat treatment is performed at a higher temperature of800° C. or more and for not less than 0.5 hours because a temperaturebelow 800° C. or a time shorter than 0.5 hours fails to sufficientlyprecipitate a fine twin structure and fine Co₃Mo or Co₇Mo₆ in boundariesbetween the fine twin structure and a parent phase. Furthermore, theaging heat treatment is performed at a temperature not more than 950° C.and for not more than 16 hours because a temperature higher than 950° C.or a time longer than 16 hours results in saturation and makes theprecipitates solve or become coarse, thereby causing decrease instrength.

[0054] In one example of the production method of the precipitationhardened Co—Ni based heat resistant alloy of the present invention, thealloy is melted and prepared through a typical method by using a vacuumhigh-frequency induction furnace, etc., and is forged into an ingotthrough a typical forging method. In one example, thereafter, the ingotis subjected to a hot working and solid solution heat treatment at 1000to 1200° C., and the ingot is then subjected to an aging heat treatmentby heating for 0.5 to 16 hours at a temperature of 600 to 800° C. in acondition of applying stress of 100 to 140 MPa. In another example,thereafter, the alloy is subjected to a cold working or warm workinghaving a reduction ratio of not less than 40% after the above-mentionedsolid solution heat treatment, and then the alloy is subjected to anaging heat treatment by heating for 0.5 to 16 hours at a temperature of600 to 800° C. in a condition of applying stress of 100 to 140 MPa. Inanother example, thereafter, the alloy is subjected to a cold working orwarm working having a reduction ratio of not less than 40% after theabove-mentioned solid solution heat treatment, and then the alloy issubjected to an aging heat treatment by heating for 0.5 to 16 hours at atemperature of 800° C. to 950° C.

[0055] The precipitation hardened Co—Ni based heat-resistant alloys ofthe present invention may be applied to parts and devices such asexhaust-related parts such as engine exhaust manifolds, peripheraldevices of gas turbines, furnace chamber materials, heat-resistantsprings and heat-resistant bolts, for which Inconel X750 or Inconel X718has been used. They may also be used for parts and devices used underhigher temperatures. Specifically, they may be preferably applied tosprings and bolts in which stress is usually applied in hightemperatures.

EXAMPLES

[0056] The following description will discuss the present inventionbased upon examples.

Example 1

[0057] Alloys of examples of the present invention and comparativeexamples, which have the compositions shown in the following Table 1,were melted and prepared through a typical method by using a vacuumhigh-frequency induction furnace to obtain ingots of 50 kg. These ingotswere formed into cylindrical bars each having a diameter of 20 mmthrough a hot forging process. Those bars were subjected to a solutionheat treatment at 1100° C., and then to an aging heat treatment of 720°C.×8 hours at a tensile stress of 200 MPa. Tensile test pieces having adiameter of 8 mm at parallel portions were obtained from these elements,and these were subjected to tensile tests at room temperature to measuretensile strength. In addition, creep test pieces having a diameter of 6mm at parallel portions with a distance between scores of 30 mm wereobtained, and these were subjected to creep tests in which a stress of330 MPa was applied thereto at 700° C. to measure the elongation 1000hours later. Table 2 shows the results of these tests. Table 2 shows theobservation result of precipitates as a microstructure. TABLE 1 (mass %)No. C Si Mn Ni Cr Mo W Mo + 1/2 W Nb Fe Ti REM B Mg Zr Co Examples 10.03 0.1 0.3 38.3 19.2 12.3 1.2 12.9 0.8 2.4 — — 0.003 — — Balance ofthe 2 0.01 0.2 0.4 42.5 21.0 17.8 — 17.8 1.1 0.9 0.6 — — 0.002 — BalancePresent 3 0.03 0.2 0.7 32.4 16.8 13.7 0.6 14.0 0.3 0.6 2.3 — 0.003 —0.02 Balance Invention 4 0.02 0.1 0.2 27.8 18.3 11.5 — 11.5 1.4 1.6 0.5— 0.002 0.001 — Balance 5 0.01 0.3 0.3 30.9 20.1 8.9 2.8 10.3 2.4 4.6 —0.02 0.005 — — Balance 6 0.02 0.4 0.2 38.6 19.1 14.9 — 14.9 1.0 1.7 0.8— 0.003 — 0.03 Balance 7 0.02 0.2 0.1 35.0 13.9 15.4 — 15.4 2.3 2.1 —0.02 — 0.003 0.04 Balance Comparative 1 0.01 0.3 0.2 39.4 15.6 8.1 — 8.11.9 2.1 — — 0.002 — — Balance Examples 2 0.02 0.2 0.3 40.9 22.4 7.6 0.88.0 0.5 1.6 1.0 — 0.001 — — Balance 3 0.01 0.4 0.6 29.7 19.1 9.0 — 9.02.2 3.8 0.4 — — 0.003 — Balance 4 0.01 0.1 0.5 33.3 18.5 7.9 0.5 8.15 —— 2.1 0.01 0.003 — 0.02 Balance

[0058] TABLE 2 Tensile Creep elongation strength at 1000 hoursPrecipitations room later (%) by aging temperature Conditions: No.treatment (MPa) 700° C. 330 MPa Examples 1 CO₇Mo₆ 1217 2.3 of the 2CO₇Mo₆ 1303 2.0 Present 3 CO₇Mo₆ 1240 2.2 Invention 4 CO₇Mo 1121 2.7 5CO₇Mo 1144 2.7 6 CO₇Mo₆ 1252 2.2 7 CO₇Mo₆ 1299 2.1 Comparative 1 —  895rupture Examples 2 —  881 rupture 3 —  976 rupture 4 —  924 rupture

Example 2

[0059] Cylindrical bars having a diameter of 20 mm of No. 5 and No. 6alloy of the present invention shown in Table 1 were subjected to asolid solution heat treatment at 1100° C. Then, as examples of thepresent invention, the cylindrical bars were subjected to an agingheating treatment of 620° C.×15 hours at a tensile stress of 250 MPa, anaging heat treatment of 720° C.×8 hours at a tensile stress of 200 MPa,or an aging heat treatment of 770° C.×4 hours at a tensile stress of 120MPa. As comparative examples, the cylindrical bars were subjected to anaging heating treatment of 850° C.×4 hours at a tensile stress of 80MPa, or an aging heat treatment of 550° C.×15 hours at a tensile stressof 250 MPa. Creep test pieces were obtained from these elements in thesame manner as in Example 1, and creep tests were carried out under thesame conditions as in Example 1 to measure creep. Table 3 shows theresults of the tests. TABLE 3 Applied stress in aging heat Aging heatCreep elongation 1000 hours treatment treatment later (%) No. Usedalloys (MPa) conditions Conditions: 700° C. 330 MPa Examples  8 Example5 of the 250 620° C. × 15 hr 2.6 of the present Present inventionInvention  9 Example 5 of the 200 720° C. × 8 hr  2.7 present invention10 Example 5 of the 120 770° C. × 4 hr  2.9 present invention 11 Example6 of the 250 620° C. × 15 hr 2.0 present invention 12 Example 6 of the200 720° C. × 8 hr  2.2 present invention 13 Example 6 of the 120 770°C. × 4 hr  2.4 present invention Comparative  5 Example 5 of the  80850° C. × 4 hr  rupture Examples present invention  6 Example 6 of the250 550° C. × 15 hr 4.6 present invention

Example 3

[0060] Cylindrical bars having a diameter of 20 mm of No. 5 and No. 6alloy of the present invention shown in Table 1 were subjected to asolid solution heat treatment at 1100° C. Then, as examples of thepresent invention, the cylindrical bars were subjected to a cold workingat reduction ratios of 45, 60 or 75%, and were then subjected to anaging heat treatment under conditions shown in Table 4 (applied stress,heating temperature and heating time). As a comparative example, thecylindrical bars were subjected to a cold working at a reduction ratioof 45%, and were then subjected to an aging heat treatment of 720° C.×8hours in an unloaded condition. Moreover, as another comparativeexample, the cylindrical bars were subjected to a cold working at areduction ratio of 60%, and were then subjected to an aging heattreatment of 720° C.×8 hours in an unloaded condition. Creep test pieceswere obtained from these elements in the same manner as in Example 1,and creep tests were carried out under the same conditions as in Example1 to measure creep. Table 4 shows the results of the tests. TABLE 4 ColdApplied stress working in aging heat Aging heat Creep elongation ratiotreatment treatment 1000 hours later No. Alloys used (%) (MPa)conditions (%) Examples 14 Example 5 of 45 400 720° C. × 8 hr 1.8 of thethe present Present invention Invention 15 Example 5 of 45 350 770° C. ×4 hr 1.9 the present invention 16 Example 5 of 60 400 700° C. × 8 hr 1.3the present invention 17 Example 5 of 60 350 720° C. × 4 hr 1.5 thepresent invention 18 Example 5 of 75 400 650° C. × 8 hr 1.0 the presentinvention 19 Example 5 of 75 350 650° C. × 4 hr 1.2 the presentinvention 20 Example 6 of 45 400 650° C. × 8 hr 1.0 the presentinvention 21 Example 6 of 60 400 650° C. × 8 hr 0.9 the presentinvention 22 Example 6 of 75 400 650° C. × 8 hr 1.2 the presentinvention Comparative  7 Example 5 of 45 — 700° C. × 4 hr 4.8 Examplesthe present invention  8 Example 5 of 60 — 720° C. × 8 hr 4.6 thepresent invention

Example 4

[0061] Cylindrical bars having a diameter of 20 mm of No. 5 and No. 6alloy of the present invention shown in Table 1 were subjected to asolid solution heat treatment at 1100° C. Then, as examples of thepresent invention, the cylindrical bars were subjected to a cold workingat reduction ratios of 60 or 75%, and were then subjected to an agingheat treatment of 850° C.×4 hours or 920° C.×2 hours in an unloadedcondition. As a comparative example, the cylindrical bars were subjectedto a cold working at a reduction ratio of 35%, and were then subjectedto an aging heat treatment of 920° C.×2 hours in an unloaded condition.Moreover, as another comparative example, the cylindrical bars weresubjected to a cold working at a reduction ratio of 75%, and were thensubjected to an aging heat treatment of 990° C.×2 hours in an unloadedcondition. Creep test pieces were obtained from these elements in thesame manner as in Example 1, and creep tests were carried out under thesame conditions as in Example 1 to measure creep. Table 5 shows theresults of the tests. TABLE 5 Cold Applied stress working in aging heatAging heat Creep elongation ratio treatment treatment 1000 hours laterNo. Alloys used (%) (MPa) conditions (%) Examples 23 Example 5 of 60 —850° C. × 4 hr 1.7 of the the present Present invention Invention 24Example 5 of 60 — 920° C. × 2 hr 1.9 the present invention 25 Example 5of 75 — 850° C. × 4 hr 1.4 the present invention 26 Example 5 of 75 —920° C. × 2 hr 1.5 the present invention 27 Example 6 of 60 — 920° C. ×4 hr 1.7 the present invention 28 Example 6 of 75 — 850° C. × 2 hr 1.3the present invention Comparative  9 Example 5 of 35 — 920° C. × 2 hr4.6 Examples the present invention 10 Example 5 of 75 — 990° C. × 2 hrrupture the present invention

[0062] According to the above-mentioned results, in the Examples No. 1to 7 of the present invention (Table 2), fine twin structure was formedwhen the structures of test pieces were observed by a SEM (scanningelectron microscope). Moreover, Co₇Mo₆ or Co₃Mo was precipitated inboundaries between the fine twin structure and a parent phase.Furthermore, the tensile strength at room temperature was set in a rangeof 1121 to 1303 MPa, and the creep elongation was 2.0 to 2.7%.

[0063] In contrast, in the case of Comparative Examples 1 to 3 in whichthe Mo+½W content was less than that of the present invention, and inthe case of Comparative Example 4 in which the Mo+½W content was lessthan that of the present invention and Nb and Fe was not contained,Co₇Mo₆ or Co₃Mo was not precipitated, the tensile strength at roomtemperature was set in a range of 881 to 976 MPa, that is, 87% of thatof the present invention, and all test pieces were ruptured in the creeptest.

[0064] In the Example No. 8 to 13 of the present invention (Table 3),fine twin structure was formed when the structures of test pieces wereobserved by a SEM (scanning electron microscope). Moreover, Co₃Mo orCo₇Mo₆ was precipitated in boundaries between the fine twin structureand a parent phase. Furthermore, the creep elongation in the creep testwas 2.0 to 2.9%.

[0065] In contrast, in the case of Comparative Example 5 in which thetemperature of the aging heat treatment was higher than that of thepresent invention, and in the case of Comparative Example 6 in which thetemperature of the aging heat treatment was lower than that of thepresent invention, Co₃Mo or Co₇Mo₆ was not precipitated, test pieceswere ruptured in the creep test in the Comparative Example 5, and thecreep elongation in the creep test was 4.6% in the Comparative Example6, that is, improvement of creep strength was not observed.

[0066] In the Examples No. 14 to 22 of the present invention (Table 4),fine twin structure was formed when the structures of test pieces wereobserved by a SEM (scanning electron microscope). Moreover, Co₃Mo orCo₇Mo₆ was precipitated in boundaries between the fine twin structureand a parent phase. FIG. 1 and FIG. 2 show structure photographs of theExample No. 22 of the present invention. By these structuralmicrographs, structure of Example No. 22 of the present invention was astructure in which massive Co₃Mo or Co₇Mo₆ was precipitated inboundaries between a fine twin structure of equilateral triangle and aparent phase. Furthermore, the creep elongations in the creep test inthe Examples No. 14 to 22 of the present invention were 0.9 to 1.9%.These creep elongations were smaller than that of the ComparativeExamples No. 7 to 13 in which cold working or a warm working having areduction ratio of not less than 40% was not performed before the agingheat treatment.

[0067] In contrast, in the case of Comparative Examples 7 and 8 in whichthe aging heat treatment was performed in an unloaded condition, Co₃Moor Co₇Mo₆ was not precipitated, and the creep elongations in the creeptests were respectively 4.8% and 4.6%, that is, improvements creepstrength were not observed.

[0068] In the Examples No. 23 to 28 of the present invention (Table 5),fine twin structure was formed when the structure of test pieces wasobserved by a SEM (scanning electron microscope). Moreover, Co₃Mo orCo₇Mo₆was precipitated in boundaries between the fine twin structure anda parent phase. Furthermore, the creep elongations in the creep tests inthe Examples No. 23 to 28 of the present invention were 1.3 to 1.9%,that is, almost equivalent to those of the Examples No. 14 to 22 of thepresent invention (Table 4; Example 3).

[0069] In contrast, in the case of Comparative Example 9 in which thecold reduction ratio was lower than that of the present invention, Co₃Moor Co₇Mo₆ was not precipitated, and the creep elongation in the creeptest was 4.6%, that is, improvement of creep strength was not observed.Moreover, in the case of Comparative Example 10 in which the temperatureof the aging heat treatment was higher than that of the presentinvention, test pieces ruptured in the creep test. Recrystallizationstructure was formed when the structure of test pieces was observed,confirming disappearance of the fine twin structure and theprecipitates.

What is claimed is:
 1. A precipitation hardened Co—Ni based heat-resistant alloy comprising by weight: not more than 0.05% of C; not more than 0.5% of Si; not more than 1.0% of Mn; 25 to 45% of Ni; 13 to 22% of Cr; 10 to 18% of Mo or 10 to 18% of Mo+½W; 0.1 to 5.0% of Nb; 0.1 to 5.0% of Fe; at least one kind of 0.007 to 0.10% of REM; 0.001 to 0.010% of B; 0.0007 to 0.010% of Mg and 0.001 to 0.20% of Zr; the balance of Co and inevitable impurities; a fine twin structure; a parent phase; and Co₃Mo or Co₇Mo₆ precipitated at boundaries of the fine twin structure and the parent phase.
 2. A precipitation hardened Co—Ni based heat-resistant alloy comprising by weight: not more than 0.05% of C; not more than 0.5% of Si; not more than 1.0% of Mn; 25 to 45% of Ni; 13 to 22% of Cr; 10 to 18% of Mo or 10 to 18% of Mo+½W; 0.1 to 5.0% of Nb; 0.1 to 5.0% of Fe; 0.1 to 3.0% of Ti; at least one kind of 0.007 to 0.10% of REM; 0.001 to 0.010% of B; 0.0007 to 0.010% of Mg and 0.001 to 0.20% of Zr; the balance of Co and inevitable impurities; a fine twin structure; a parent phase; and Co₃Mo or Co₇Mo₆ precipitated at boundaries of the fine twin structure and the parent phase.
 3. A production method for precipitation hardened Co—Ni based heat-resistant alloy, the method comprising the steps of: preparing an alloy comprising by weight: not more than 0.05% of C; not more than 0.5% of Si; not more than 1.0% of Mn; 25 to 45% of Ni; 13 to 22% of Cr; 10 to 18% of Mo or 10 to 18% of Mo+½W; 0.1 to 5.0% of Nb; 0.1 to 5.0% of Fe; at least one kind of 0.007 to 0.10% of REM; 0.001 to 0.010% of B; 0.0007 to 0.010% of Mg and 0.001 to 0.20% of Zr; and the balance of Co and inevitable impurities; subjecting the alloy to a solid solution heat treatment; and subjecting the alloy to an aging heat treatment at 600 to 800° C. for 0.5 to 16 hours in a condition of applying stress, thereby forming a fine twin structure in a parent phase, and precipitating Co₃Mo or Co₇Mo₆ at a boundary of the fine twin structure and the parent phase.
 4. A production method for precipitation hardened Co—Ni based heat-resistant alloy, the method comprising the steps of: preparing an alloy comprising by weight: not more than 0.05% of C; not more than 0.5% of Si; not more than 1.0% of Mn; 25 to 45% of Ni; 13 to 22% of Cr; 10 to 18% of Mo or 10 to 18% of Mo+½W; 0.1 to 5.0% of Nb; 0.1 to 5.0% of Fe; 0.1 to 3.0% of Ti; at least one kind of 0.007 to 0.10% of REM; 0.001 to 0.010% of B; 0.0007 to 0.010% of Mg and 0.001 to 0.20% of Zr; and the balance of Co and inevitable impurities; subjecting the alloy to a solid solution heat treatment; and subjecting the alloy to an aging heat treatment at 600 to 800° C. for 0.5 to 16 hours in a condition of applying stress, thereby forming a fine twin structure in a parent phase, and precipitating Co₃Mo or Co₇Mo₆ at a boundary of the fine twin structure and the parent phase.
 5. A production method for precipitation hardened Co—Ni based heat-resistant alloy, the method comprising the steps of: preparing an alloy comprising by weight: not more than 0.05% of C; not more than 0.5% of Si; not more than 1.0% of Mn; 25 to 45% of Ni; 13 to 22% of Cr; 10 to 18% of Mo or 10 to 18% of Mo+½W; 0.1 to 5.0% of Nb; 0.1 to 5.0% of Fe; at least one kind of 0.007 to 0.10% of REM; 0.001 to 0.010% of B; 0.0007 to 0.010% of Mg and 0.001 to 0.20% of Zr; and the balance of Co and inevitable impurities; subjecting the alloy to a solid solution heat treatment; subjecting the alloy to a cold working or a warm working having a reduction ratio of not less than 40%; and subjecting the alloy to an aging heat treatment at 600 to 800° C. for 0.5 to 16 hours in a condition of applying stress, thereby forming a fine twin structure in a parent phase, and precipitating Co₃Mo or Co₇Mo₆ at a boundary of the fine twin structure and the parent phase.
 6. A production method for precipitation hardened Co—Ni based heat-resistant alloy, the method comprising the steps of: preparing an alloy comprising by weight: not more than 0.05% of C; not more than 0.5% of Si; not more than 1.0% of Mn; 25 to 45% of Ni; 13 to 22% of Cr; 10 to 18% of Mo or 10 to 18% of Mo+½W; 0.1 to 5.0% of Nb; 0.1 to 5.0% of Fe; 0.1 to 3.0% of Ti. at least one kind of 0.007 to 0.10% of REM; 0.001 to 0.010% of B; 0.0007 to 0.010% of Mg and 0.001 to 0.20% of Zr; and the balance of Co and inevitable impurities; subjecting the alloy to a solid solution heat treatment; subjecting the alloy to a cold working or a warm working having a reduction ratio of not less than 40%; and subjecting the alloy to an aging heat treatment at 600 to 800° C. for 0.5 to 16 hours in a condition of applying stress, thereby forming a fine twin structure in a parent phase, and precipitating Co₃Mo or Co₇Mo₆ at a boundary of the fine twin structure and the parent phase.
 7. A production method for precipitation hardened Co—Ni based heat-resistant alloy, the method comprising the steps of: preparing an alloy comprising by weight: not more than 0.05% of C; not more than 0.5% of Si; not more than 1.0% of Mn; 25 to 45% of Ni; 13 to 22% of Cr; 10 to 18% of Mo or 10 to 18% of Mo+½W; 0.1 to 5.0% of Nb; 0.1 to 5.0% of Fe; at least one kind of 0.007 to 0.10% of REM; 0.001 to 0.010% of B; 0.0007 to 0.010% of Mg and 0.001 to 0.20% of Zr; and the balance of Co and inevitable impurities; subjecting the alloy to a solid solution heat treatment; subjecting the alloy to a cold working or a warm working having a reduction ratio of not less than 40%; and subjecting the alloy to an aging heat treatment at 800° C. to 950° C. for 0.5 to 16 hours, thereby forming a fine twin structure in a parent phase, and precipitating Co₃Mo or Co₇Mo₆ at a boundary of the fine twin structure and the parent phase.
 8. A production method for precipitation hardened Co—Ni based heat-resistant alloy, the method comprising the steps of: preparing an alloy comprising: all by weight, not more than 0.05% of C; not more than 0.5% of Si; not more than 1.0% of Mn; 25 to 45% of Ni; 13 to 22% of Cr; 10 to 18% of Mo or 10 to 18% of Mo+½W; 0.1 to 5.0% of Nb; 0.1 to 5.0% of Fe; 0.1 to 3.0% of Ti; at least one kind of 0.007 to 0.10% of REM; 0.001 to 0.010% of B; 0.0007 to 0.010% of Mg and 0.001 to 0.20% of Zr; and the balance of Co and inevitable impurities; subjecting the alloy to a solid solution heat treatment; subjecting the alloy to a cold working or a warm working having a reduction ratio of not less than 40%; and subjecting the alloy to an aging heat treatment at 800° C. to 950° C. for 0.5 to 16 hours, thereby forming a fine twin structure in a parent phase, and precipitating Co₃Mo or Co₇Mo₆ at a boundary of the fine twin structure and the parent phase. 